1. Field of the Invention
The present invention relates to devices which measure tissue blood flow, particularly those based on the thermal diffusion flow concept, measure tissue pressure and can also assess the function of human tissue through the simultaneous monitoring of critical physiological parameters.
2. Description of the prior art
The original reports using the thermal diffusion flow monitor concept appeared in the late 1960's. The work was done by Carter et al. (Carter L. P., Atkinson J. R. "Cortical blood flow in controlled hypotension as measured by thermal diffusion", J. Neurol. Neurosurg. Psychiatry, Vol. 36, pp. 906-913, 1973) using a Peltier stack. In order for the Peltier stack to be able to detect flow (as determined by the rate of cooling) the tissue of interest needed to be exposed and uniform contact between the sensor and the tissue surface was required. Although Peltier stacks are widely used, the system suffers from its relatively large size of the sensor and the variability of its output.
Later, in the 1980's, improvements including signal processing to stabilize the sensors output, simplified design of the sensor using a two-point system, where one point is a heat source and the other point a temperature sensor being positioned a short distance away, were made. Still because of its large size , need for visual placement of the probe on the surface of the tissue and unreliable readings due to loose contact of the sensor tip with the surface of the tissue continue to limit its application.